This application is based on Application No. 2001-153056, filed in Japan on May 22, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an O2-sensor fault diagnosis apparatus and method for diagnosing whether a fault occurs in an O2-sensor used to perform feedback control of a fuel supply to an internal combustion engine.
2. Related Background Art
O2-sensors are used to perform air/fuel ratio feedback control for internal combustion engines, as described in Japanese Patent Application Laid-open No. Sho 57-137633. Also, various O2-sensor fault diagnosis apparatuses have been conventionally proposed which detect faults of O2-sensors and breaking of signal wires on the basis of output voltages of the sensors.
Such O2-sensors, however, have a problem in that irrespective of air/fuel ratios, their output voltages remain low until they become active and thus it is difficult to distinguish this situation from a state where there is breaking of signal wires. A conventional fault diagnosis apparatus is disclosed in Japanese Patent Application Laid-open No. Hei 5-203611, according to which if an inactive state of an O2-sensor is detected, air/fuel ratio is forcibly enriched. If the O2-sensor remains in the inactive state even after the air/fuel ratio is enriched, the sensor is diagnosed as having a fault. With this technique, however, the enriched air/fuel ratio causes the increase of pollutants in an exhaust gas and misdiagnosis is made depending on the amount of injected fuel.
Also, proposed is another conventional diagnosis apparatus that precisely detects a state where there is wire breaking by changing the input resistance of an input circuit to an ECU that is connected to an O2-sensor. With this technique, however, feedback control needs to be temporarily suspended when the input resistance is changed. This portion that frequent input resistance change increases pollutant emissions, so that once the detection of wire breaking is carried out, it is difficult to conduct fault diagnosis again. As a result, even if wire breaking occurs during driving after the detection of wire breaking is performed, it is impossible to inform a driver of the necessity of repair at an early stage.
As described above, with the conventional O2-sensor fault diagnosis apparatuses, it is difficult to precisely distinguish an inactive state from a state where wire breaking occurs and at the same time, to successively perform the detection of wire breaking. As can be seen from this, there is still room for improvement in the O2-sensor fault diagnosis apparatuses.
The present invention has been made to solve the stated problems and an object of the present invention is to achieve an O2-sensor fault diagnosis apparatus and method therefor, which enable successive detection of wire breaking without increasing pollutant emissions.
An O2-sensor fault diagnosis apparatus according to this invention comprises: an O2-sensor for detecting concentration of oxygen contained in an exhaust gas of an internal combustion engine; a feedback control portion for controlling a quantity of fuel supplied to the internal combustion engine through feedback control according to an output signal of the O2-sensor; a state judging portion for judging whether the O2-sensor is in an active state or in an inactive state on the basis of an voltage of the output signal of the O2-sensor; and a fault diagnosis portion for diagnosing whether the O2-sensor has any fault on the basis of the voltage of the output signal of the O2-sensor under a condition where it is judged that the O2-sensor is in the inactive state.
Also, the fault diagnosis portion includes an input resistance changing portion for changing an input resistance so as to cause a change in a level of the output signal of the O2-sensor, and identifies a fault of the O2-sensor on the basis of the change in the level of the output signal caused by changing the input resistance.
Further, the fault diagnosis portion diagnoses whether the O2-sensor has any fault each time the state judging portion judges that the O2-sensor is in the inactive state.
Furthermore, the O2-sensor fault diagnosis apparatus according to this invention further comprises an informing portion for sending a notice if the fault diagnosis portion diagnoses that the O2-sensor has a fault.
Also, an O2-sensor fault diagnosis method according to this invention comprises: a state judging step for judging whether an O2-sensor, which detects concentration of oxygen contained in an exhaust gas of an internal combustion engine, is in an active state or in an inactive state on the basis of an voltage of an output signal of the O2-sensor; and a fault diagnosis step for diagnosing whether the O2-sensor has any fault on the basis of the voltage of the output signal of the O2-sensor under a condition where it is judged that the O2-sensor is in the inactive state.
Further, in the fault diagnosis step, a fault of the O2-sensor is identified on the basis of a change in a level of the output signal of the O2-sensor caused by changing an input resistance.
Furthermore, in the fault diagnosis step, it is diagnosed whether the O2-sensor has any fault each time it is judged in the state judging step that the O2-sensor is in the inactive state.
Finally, the O2-sensor fault diagnosis method according to this invention further comprises an informing step for sending a notice if the O2-sensor is diagnosed to have a fault in the fault diagnosis step.